PaX/GrSecurity Linux 3.8.y compat: Use __no_const on struct ctl_table

The PaX team started constifying `struct ctl_table` as of their Linux
3.8.0 patchset. This lead to zfsonlinux/spl#225 and Gentoo bug #463012.

While investigating our options, I learned that there is a preprocessor
directive called CONSTIFY_PLUGIN that we can use to detect the presence
of the PaX changes and adjust the code accordingly.

The PaX Team had suggested adopting ctl_table_no_const, but supporting
older kernels required declaring that whenever the CONSTIFY_PLUGIN was
set. Future compiler changes could potentially cause that to break in
the presence of -Werror, so instead we define our own spl_ctl_table
typdef and use that. This should be compatible with all PaX kernels.

Note that this introduces a Linux version number check. This is
necessary because a build failure will occur on older versions of the
PaX patch set that existed for kernels before Linux 3.8.0.i Affected
versions of the PaX plugin will trigger a compiler error when they see
no_const cast on a non-constified structure.  Ordinarily, we would need
an autotools check to catch that. However, it is safe to do a kernel
version check instead of an autotools check in this specific instance
because the affected versions of the PaX GCC plugin only exist for Linux
kernels before 3.8.0 and the constification of `struct ctl_table` by the
PaX developers only occurs in Linux 3.8.0 and later.

Closes zfsonlinux/spl#225

Signed-off-by: Richard Yao <ryao@gentoo.org>